Nengmy Vang, 45, died at around 1:30 AM, state Department of Justice spokesman Johnny Koremenos said.
Vang had been hospitalised since March 22, when police shot him during a standoff at his apartment in Weston, one of a cluster of small towns near Wausau, about 90 miles west of Green Bay.
Investigators say Vang barricaded himself in the apartment after killing three people in a rampage that spanned three cities.
The Wisconsin DOJ is handling the investigation and has released few details, and Koremenos said no update on the investigation would be given today. However, investigators said Vang launched his attack after a domestic incident with his wife. The couple were going through a bitter divorce.
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He then fled to his apartment and barricaded himself inside. Investigators say he shot and killed Everest Metro Police Detective Jason Weiland as Weiland was setting up a perimeter around the apartment. A standoff ensued before police shot Vang and took him into custody.
Vang's wife was not harmed. Investigators have not said what precipitated the domestic incident, what happened during that incident or whether she was present at the bank at the time of the attack. They also hadn't officially identified Vang until they announced his death.
Online court records show Vang was struggling with debt in addition to his marital problems. His brother, Vajloogzeb Vaj, told The Associated Press that Vang had shown signs of mental illness since he and his wife separated and that Vang had once struck their mother. Vaj said he hadn't spoken with his brother in weeks.